Ferrofluid exclusion seals find application in products such as computer disk drive assemblies where it is necessary to prevent contaminants from passing through the gap between two members which are being rotated relative to each other. For example, in disk drive assemblies, it is necessary to isolate the disk cavity to prevent contaminants from the drive motor and bearings and from the external environment from reaching the cavity along the disk drive shaft.
Prior art patents describing such seals include U.S. Pat. No. 4,357,021 issued to Raj et al on Nov. 2, 1982, and U.S. Pat. No. 4,694,213, issued to Gowda et al on Sept. 15, 1987. Both of these patents show ferrofluid seals wherein the ferrofluid is retained in a gap formed on one side by a magnet and at least one pole piece, and on the other side by a spindle or shaft, and wherein, for various reasons, the gap tapers in the area adjacent all or a portion of one of the pole pieces. This is typically accomplished by chamfering the pole piece. The magnet may be recessed from the pole piece. However, with the trend toward miniaturization of equipment such as disk drives, such equipment has become thinner and thinner, thereby severely limiting the axial space available for ferrofluid seals and therefore requiring designs with very small axial length or thickness. In some such applications, the available axial length may be less than 0.050 inches. However, as with most fluids, ferrofluid evaporates with time. Thus, as the axial width of the ferrofluid seal decreases, decreasing the space in the seal available for ferrofluid, there may be a corresponding decrease in the useful life of the seal. A need therefore exists for a relatively simple technique for increasing the amount of ferrofluid available in seals having a short axial length, so that the useful life of the seal is at least as large as that of other components in the disk drive or other equipment in which the seal is utilized.
Further, in an effort to maximize the ferrofluid in the gap, the fluid frequently fills the entire gap, resulting in a bulge above the level of the pole piece, which may result in ferrofluid splash in operation. Ferrofluid expansion due to temperature changes may also result in ferrofluid outside the magnetic field region and thus in splash. It would therefore be desirable if a means could be provided to permit sufficient ferrofluid to be used in such small-sized applications without requiring that the ferrofluid fill the seal gap to a point where splash or spillover may occur.
Another problem with many existing seals is that the uniform width of the gap results in very sharp changes in the magnetic field at the gap edges. Since the magnetic particles in the ferrofluid tend to migrate to the areas of high magnetic field, these changes result in separation of magnetic particles in the ferrofluid which adversely affects the effectiveness of the seal. The high gradients, and in particular the separation caused thereby, cause the ferrofluid properties to change with time and normally result in a progressive decrease in static pressure capacity of the seal. A ferrofluid seal design which reduces or minimizes such separation is therefore desirable.
A final problem with existing ferrofluid seals of the type described in the prior patents is that it is frequently difficult to form a desired taper on a pole piece which is very thin; for example, a pole piece having a width in the range of 0.03 inches.
Thus, as the need arises for ferrofluid seals having smaller axial widths, improvements in such seals are required (a) to permit such seals to be more easily manufactured; (b) to increase the amount of ferrofluid in such a seal for a given seal width, thus increasing the seal life; (c) to permit the ferrofluid level in the gap to be low enough so as to avoid bulge and overflow, thereby eliminating splash and other loss of ferrofluid; and (d) to make the magnetic field gradient more uniform so as to minimize separation in the ferrofluid, thus eliminating decreases in static pressure capacity with time.